The two principal methods of producing hydrogenated amorphous silicon are the glow discharge decomposition of silane and reactive sputtering in a mixture of argon and hydrogen. Hydrogenated amorphous silicon films produced by these techniques are known to have differences in optical and electronic properties. Those of the art attribute some these distinctions to the differences in the bonding configuration of hydrogen into the amorphous silicon network. For example, when hydrogen bonds in polyhydride configurations, it introduces electronic defects in the gap of the semiconductor and also influences the optical gap of the material by modifying the valence and possibly the conduction bands. In view of these observations, previous efforts have attempted to identify the deposition parameters which controls the bonding of hydrogen into the amorphous network.
The silicon-hydrogen bonding characteristics in photoconductive amorphous silicon is conventionally explained in relation to the local environments of the incorporated hydrogen. Those of the art believe that the hydrogen can be present either in the monohydride SiH form, isolated dihydride SiH.sub.2 units, coupled dihydride SiH.sub.2 units, higher order polyhydrides such as SiH.sub.3, or combinations thereof. For purposes of the present application, the term polyhydride will be used to collectively define all silicon-hydrogen bonding forms other than the monohydride, SiH. Typically, infrared absorbtion or vibration spectra data is used to analyze the bonding characteristics of the silicon film. An analysis of the silicon-hydrogen vibrational spectra indicates the density of silicon to hydrogen bonding of the monohydride and polyhydride forms.
Referring momentarily to the drawings, an illustration of an infrared absorption spectra is shown in FIG. 2. The three absorption bands are identified as the stretching (.about.2000cm.sup.-1), bending (.about.900cm.sup.-1) and wagging (.about.600cm.sup.-1) modes of SiH and SiHx groupings. The presence of polyhydrides in the film is evidenced by the bending modes, whose origin are forces opposing changes in the angles between Si--H bonds within SiH.sub.2 and SiH.sub.3 groups. Contrary to the stretching and wagging vibrations, the bending modes are absent when the hydrogen is bonded only in monohydride configurations. For purposes of the present invention, the bonding mode evidenced at 2000 cm.sup.-1 is the stretching vibration of monohydride groups and the bonding mode evidenced at 2100 cm.sup.-1 is the stretching vibration of the polyhydride groups. The integrated absorption under these two modes provides a measure of the density of monohydride and polyhydride bonds respectively. The present invention teaches deposition techniques for producing reactively sputtered photoconductive amorphous silicon films having controlled monohydride and polyhydride bond densities. A positive or negative DC voltage bias applied to the substrates during the sputter deposition increases or decreases the relative density of monohydride to polyhydride bonds in the resultant film.
The ability to control the relative density of the monohydride and polyhydride bonds afforded by the present invention, further provides the ability to control the optical properties of the resultant film and more particularly the optical band gap. This ability is generally considered an important advantage in optimizing the photovoltaic characteristics of amorphous silicon hydride.
The art has demonstrated the ability to control the relative density of monohydride and polyhydride bonds in amorphous silicon produced by glow discharge decomposition of silane. For example, Brodsky et al in a technical--publication entitled "Infrared & Raman spectra of the silicon-hydrogen bonds in amorphous silicon prepared by glow discharge and sputtering" teach the basic glow discharge deposition parameters which control the bonding of hydrogen. Films produced at low silane pressure (.about.0.1 monohydrides (The stretching vibration is centered around 2000 cm.sup.-1). Films produced at low temperatures (Ts.about.25.degree. C.) and higher silane pressure (.about.1 Torr) contain primarily polyhydrides (The stretching vibration is centered around 2100 cm.sup.-1).
Predetermined control of the bonding of hydrogen in sputtered hydrogenated amorphous silicon has not been previously achieved in the art. Indeed Brodsky et al, in Physical Review, B. 16, No. 8, 10/77 and Freeman et al, in Physical Review B. 18, No. 8, 10/15/78, teach that for sputter deposited amorphous silicon the stretching vibration is always a doublet, which together with the existence of the bending mode at 900 cm.sup.-1 teach that the sputtered films always contain a mixture of monohydride and polyhydride bonding configurations. One technique for altering the densities of the monohydride and polyhydride bonds is set forth by F. R. Jeffrey et al in J. of Applied Physics, Vol. 50 p. 7034 (1979) wherein a change in the relative density of monohydride to dihydride bonding was noted for films sputtered at different power levels.